This invention relates to integral skin foams having improved properties, and a process for the production of these foams. These foams are prepared from an isocyanate comprising a stable, liquid MDI based prepolymer containing an allophanate-modified MDI.
Liquid diphenylmethane diisocyanates and their use in the preparation of integral skin foams is generally known in the art. Various types of liquid diphenylmethane diisocyanates include, for example, isocyanate prepolymers, allophanate-group containing isocyanates, carbodiimide group containing isocyanates, biuret group containing isocyanates, etc. These are described in, for example, U.S. Pat. Nos. 3,644,457, 4,055,548, 4,115,429, 4,118,411, 4,160,080, 4,261,852, 4,490,300, 4,738,991 and 4,866,103, and GB 994,890.
Integral skin foams prepared from polyurethane are also well known in the art. These are described in, for example, U.S. Pat. Nos. 3,523,918, 3,726,952, 3,836,487, 3,925,527, 4,020,001, 4,024,090, 4,065,410, 4,305,991 and 5,166,183; and CA 1,277,471.
U.S. Pat. No. 3,644,457 discloses room temperature stable liquid isocyanates derived from one mole of diphenylmethane diisocyanate and 0.1 to 0.3 mols of poly-1,2-propylene ether glycol.
U.S. Pat. No. 4,055,548 discloses liquid isocyanate prepolymer compositions obtained by reacting polymethylene polyphenylisocyanate containing from about 65 to 85 percent by weight of methylene bis(phenyl)isocyanate with a polyoxyethylene glycol having molecular weight of from 200 to 600 in an equivalent ratio of 0.0185 to 0.15:1.
U.S. Pat. Nos. 4,115,429 and 4,118,411 disclose low temperature (as low as -5.degree. C.), storage stable liquid diphenylmethane diisocyanates which are produced by reacting diphenylmethane diisocyanates having a specified 2,4-isomer content with propylene glycol or poly-1,2-propylene ether glycol.
U.S. Pat. No. 4,261,852 discloses liquid polyisocyanate compositions comprising (a) the reaction product of 90 to 50% by weight of a reaction product of diphenylmethane diisocyanate and a polyoxypropylene diol or triol having a hydroxyl equivalent weight of from 750 to 3000, said reaction product having an NCO content of from 8 to 26% by weight, and (b) from about 10 to 50% by weight of a diphenylmethane diisocyanate containing from 30 to 65% by weight of diphenylmethane diisocyanate, the remainder being polymethylene polyphenyl polyisocyanate.
U.S. Pat. No. 4,490,300 discloses room temperature stable liquid isocyanates which are derived by reacting diphenylmethane diisocyanate with an aliphatic diol having a pendant aromatic group, e.g., 2-methyl-2-phenyl-1,3-propanediol or phenyl-1-2-ethanediol.
U.S. Pat. No. 4,490,301 discloses room temperature stable liquid isocyanates which are derived by reacting diphenylmethane diisocyanate with monoallylether of trimethylolpropane.
U.S. Pat. No. 4,738,991 discloses organic polyisocyanates characterized by allophanate linkages which are prepared by reacting an organic polyisocyanate including 2,4- and 4,4-methylenediphenyl diisocyanate with poly- or monohydric alcohol in the presence of an organometallic catalyst. The catalyst is then deactivated using a compound such as an inorganic acid, organic acid, organic chloroformate or an organic acid chloride. This reference also discloses that flexible foams can be prepared from these allophanate group containing isocyanates. All of the examples relate to TDI based allophanate group containing isocyanates, and only one of these prepares a high resilience flexible foam.
U.S. Pat. No. 4,866,103 discloses a polyisocyanate composition for use in producing elastomers in a RIM process. This polyisocyanate composition is the reaction product of an alcohol and/or thiol having an average functionality of from about 1.5 to about 4 and an average equivalent weight of at least 500 with at least 2 equivalents per hydroxyl and/or thiol equivalent of an organic polyisocyanate including 4,4- and 2,4-isomers of diphenylmethane diisocyanate. The disclosed reaction is carried out under conditions such that at least about 20% of the initially formed urethane and/or thiourethane groups are converted to allophanate and/or thio allophanate groups.
Another process for the preparation of allophanates which contain isocyanates is disclosed in British Patent 994,890 which relates to the reaction of urethane isocyanates with excess diisocyanate either by heat alone or in the presence of a catalyst such as a metal carboxylate, a metal chelate or a tertiary amine, until the isocyanate content is reduced to that which is obtained theoretically when the complete reaction of the urethane groups is achieved.
U.S. Pat. No. 4,160,080 discloses a process for producing allophanate-containing aliphatically and/or cycloaliphatically bound isocyanate groups. In this disclosed process, compounds containing urethane groups are reacted with polyisocyanates having aliphatic and/or cycloaliphatic isocyanate groups in the presence of a strong acid. The process is generally conducted at a temperature of from 90.degree. C. to 140.degree. C. for about 4 to 20 hours.
Japanese Patent Application No. 1971-99176 discloses a method of preparing liquid diphenylmethane diisocyanate by reacting diphenylmethane diisocyanate with an aliphatic monovalent alcohol.
Integral skin foams are described and prepared in U.S. Pat. No. 4,305,991. These foams are prepared from a reaction mixture containing a polyisocyanate wherein the isocyanate groups are aliphatically and/or cycloaliphatically bound. These polyisocyanates may contain adducts such as, for example, carbodiimide-, allophanate-, isocyanurate-, uretdione-, biuret-groups, etc. The aliphatic isocyanates used to prepare these foams provide resistance to UV light and to decomposition by heat.
U.S. Pat. No. 5,166,183 also describes integral skin foams. The polyisocyanate composition used therein has an NCO content of about 16 to 25%, and consists of i) from 10 to 100 parts by weight of an isocyanate having an NCO content of about 16 to 22% which is prepared by blending methylenebis(phenyl isocyanate) and a carbodiimide group modified methylenebis(phenyl isocyanate), followed by reacting with a polyester diol to form a product. This product is then blended with ii) from 0 to 90 parts by weight of a modified isocyanate having an NCO content of about 18 to 25% which is prepared by reacting methylenebis(phenyl isocyanate) with poly-1,2-propylene ether glycol. The presence of the carbodiimide-modified isocyanate in the prepolymer serves to reduce the freezing point.
The integral skin foams of the '183 patent are described as exhibiting improved abrasion resistance. This is contributed to by the use of a polyester in the prepolymer formation. Polyesters, however, add to the cost of the prepolymer and have decreased miscibility (solubility) with polyethers such that the processing of the systems is difficult.
Carbodiimide modified isocyanates are also disclosed as suitable isocyanates for the production of integral skin foams in U.S. Pat. No. 5,342,856. These isocyanates are reacted with an isocyanate-reactive component and a solution of a zinc carboxylate in an aliphatic polyamine. These solutions of zinc carboxylates in aliphatic polyamines and water are essential to the preparation of integral skin foams.
U.S. Pat. No. 4,477,602 describes a system for the production of polyurethanes with improved low temperature flex-fatigue resistance and green strength. A key component of this system is a dispersion of a polyurea and/or a polyhydrazodicarbonamide in an organic compound. The examples also employ CFC-11 as the primary blowing agent with water as a minor blowing agent.
U.S. Pat. No. 5,585,409 describes a process for producing molded polyurethanes with a cellular core and a compact surface zone. These polyurethanes are produced from a system comprising an isocyanate semi-prepolymer containing ester groups which is prepared from a polyester polyol and a branched chain dihydroxy compound containing at least one bonded ester unit.
U.S. Pat. No. 5,624,966 describes a polyurethane composition for producing polyurethane articles having a tensile strength greater than or equal to 450 psi and/or a Taber abrasion (mg loss) of less than 200.
U.S. Pat. No. 5,514,723 describes an improvement over the process disclosed in U.S. Pat. No. 5,166,183. More specifically, the '723 patent relates to the discovery that a particular combination of catalysts improves the ratio of the tensile strength of the skin to the tensile strength of the core of the integral skin foams described in the '183 patent. This catalyst combination comprises: a) a diorganotin sulfide corresponding to a specific formula, b) a tertiary amine, and c) a tin compound capable of catalyzing the reaction between an isocyanate group and an active hydrogen atom, with the proviso that c) is not a diorganotin sulfide as described in a).
Applicants' copending application Ser. No. 08/680,094, filed on Jul. 15, 1996, which is commonly assigned, relates to integral skin foams. These foams are prepared from an isocyanate comprising a stable, liquid MDI based prepolymer containing an allophanate-modified MDI. The integral skin foams of this application had superior abrasion resistance in comparison to integral skin foams made with different isocyanates. These integral skin foams did not, however, exhibit the flexural properties required for footwear applications.
Furthermore, when allophanate-modified isocyanates were used in formulations expected to yield better flexural properties, it was found that certain allophanate modified isocyanate prepolymers were superior to the allophanate modified isocyanates required in this copending application.
The improved abrasion resistance described in the above copending application would, actually, be of little advantage in the present invention, because the systems of the present invention already achieve superior abrasion resistance by other means. On the other hand, the improved flexural properties of the molded articles claimed in the present application are not obvious from, nor would these be expected from consideration of the copending application.
An object of the present invention was to develop a polyurethane integral skin system especially suited for soling or shoe sole (i.e., footwear) applications. This requires that the system possesses superior dynamic flexural fatigue properties. It was preferred that these systems use a blowing agent comprising water.
Another object was to avoid the disadvantages resulting from the use of polyester-extended prepolymers. Such disadvantages include, for example, high viscosities, high costs, and susceptibility to hydrolysis.